A small fire at the old Main Post Office over the weekend was caused by a passing or idling train that ignited built-up tar in a ventilation shaft, a Chicago Fire Department spokesman said this morning.

The blaze flared up the shaft of the long-closed building at 433 W. Van Buren St. and spread to the roof and 10th floor Friday night.

The old post office, closed in 1995, sits above portions of Union Station and its south platforms and tracks. The Fire Deparment's Office of Fire Investigations has determined that embers or flames from the stack of a train ignited tar and other byproducts that have built up in the shaft from years of trains passing and idling underneath, according to Fire Department spokesman Larry Langford.

The Fire Department, however, "reserves final judgment pending a full electrical and mechanical inspection of the ventilation system," Langford said in a statement.

Amtrak and the owner of the post office are already feuding over fumes from diesel trains. Amtrak says the fumes build up in Union Station because the building is poorly ventilated.

Earlier this month, Amtrak earlier this month sued the owner of the building -- International Property Developers North America Inc. -- claiming that “refusal to maintain its ventilation system below the Old Post Office" was exposing passengers to a heavy dose of fumes.

But International Property Developers North America said the fault was with Amtrak, blaming “trains idling for prolonged periods under the old post office.”

While a train appears to have ignited the weekend fire, Amtrak spokesman Marc Magliari noted that the allegations in the lawsuit include a lack of maintenance of the old post office ventilation system since IPD bought the building in 2009.

But Martin Mulryan, project manager for the IPD at the post office, said the ventilation system is regularly maintained. “The cleaning of soot and creosote is part of an ongoing maintenance issue and they are cleaned four times per year," Martin said in an email.

“IPD have staff at the building and are reviewing the cleaning regime this week to see if increasing the cleaning frequency would help,” he added.

However, just two days before the fire IPD “at the request of Union Station increased the ventilation ... and this increased ventilation may have helped keep burning embers alight,” he suggested.

Mulryan said trains need to “to shut down their engines. . .in the station” and eventually need to be replaced with more efficient trains with better control of flames from their stacks.